This invention pertains to the preparation of a new composition of matter containing a tetrasubstituted organosilicon moiety. More specifically, the invention relates to a process for the synthesis of an aromatic or heteroaromatic compound, in particular a thiophenol or phenol, bearing an organosilyl, preferably trialkysilyl, substituent in the ortho, meta, or para position. Such compounds are useful for formation of photographic couplers for use in photographic materials and processes.
Compounds that release a group, particularly photographic couplers that release a coupling-off group, such as those described in U.S. Pat. No. 3,935,015, are well-known in the photographic art. However, such compounds often do not have the desired degree of stability when incorporated in photographic materials, particularly during storage before exposure. Moreover, the known coupling-off groups can show insufficient reactivity and unacceptable background discoloration. Furthermore, the known photographic couplers can yield image dyes upon oxidative coupling that are insufficiently stable to light fade.
Improved couplers that overcome the foregoing disadvantages of the known couplers comprise a silyl coupling-off group that is capable of being released upon processing in a photographic silver halide material, as described in copending U.S. application Ser. No. 07/986,841 of S. Krishnamurthy, filed concurrently with the present application. Such couplers include, for example, trialkylsilylarylthio or trialkylsilylaryloxy coupling-off groups on a parent photographic coupler moiety, for instance an o-trialkylsilylarylthio group or an o-trialkylsilylaryloxy group on a parent pyrazolone coupler. Such couplers are new compounds that require a new method of preparation that enables formation of the compounds in very good yields, while avoiding the need for complicated intermediate separation and purification steps. The new method of coupler synthesis also requires the formation of the substituted silylarylthio or substituted silylaryloxy coupling-off groups by a new method that enables effective yield of the desired product.
Synthesis of trialkylsilyl substituted arylthiols and phenols via the rearrangement of O- or S-trialkylsilylarylmetal is described in the scientific literature [A. R. Bassindale and D. R. M. Walton, J. Organometal. Chem., vol. 25, p. 389 (1970); G. Somchen and J. Pfletschinger, Angew. Chem., vol. 88, p. 444 (1976); J. Heinicke, E. Nietzschmann, and A. Tzschach, J. Organometal. Chem., vol. 243, p. 1 (1983)]. However, the cited synthetic procedures often yielded unwanted side products, namely S- or O-butylated arylthiols or phenols, with the butyl group by-product originating from the n-butyl bromide, generated in the metallization step with n-butyllithium acting as an electrophile. This was found to be especially problematic during the synthesis of trialkylsilyl-substituted thiophenols, as thiophenolates are exceptionally powerful nucleophiles and rapidly react with the n-butyl bromide present. For example, carrying out the disclosed rearrangement of o-bromo(S-trimethylsilyl)benzenethiol via the metallized intermediate (generated by n-butyllithium) at room temperature without close monitoring led to the S-butylated derivative as the exclusive final product.
There thus exists a need for a convenient, efficient, and expeditious synthesis of trialkylsilylarylthiols and trialkylsilylphenols under mild conditions.